Surface & Coatings Technology 200 (2006) 6594 – 6600 www.elsevier.com/locate/surfcoat A comparative study of the structure and the corrosion behavior of zinc coatings deposited with various methods G. Vourlias a , N. Pistofidis a , D. Chaliampalias a , E. Pavlidou a , P. Patsalas b , G. Stergioudis a,⁎, D. Tsipas c , E.K. Polychroniadis a a Aristotle University of Thessaloniki, Department of Physics, GR 54124 Thessaloniki, Greece University of Ioannina, Department Materials Science and Engineering, GR 45110 Ioannina, Greece Aristotle University of Thessaloniki, Department of Mechanical Engineering, GR 54124 Thessaloniki, Greece b c Available online 13 December 2005 Abstract The structure and the corrosion performance of Zn coatings formed by pack cementation and fluidized bed reactor at 250, 350, 380 and 400 °C have been studied. From this investigation, it turned out that these coatings are composed of two layers referring to γ-Fe11Zn40 and δ-FeZn10 phases of the Fe–Zn phase diagram. Furthermore, in the pack coatings, inclusions were detected, which are composed of Fe and Zn at almost equal concentrations. Concerning the corrosion, a mechanism of stress corrosion has been identified. The corrosion performance of the studied coatings is similar to that of the hot-dip galvanized coatings, although the deposition method is different. © 2005 Elsevier B.V. All rights reserved. Keywords: Surfaces and interfaces; Coating materials; Metals and alloys; Scanning electron microscopy; X-ray diffraction 1. Introduction Atmospheric corrosion, as far as it regards ferrous materials, is one of the most important issues. The repair of the damages that this phenomenon causes costs several billions of Euros nowadays, without taking into account the environmental impact and the loss of natural resources [1,2]. Therefore, many methods have been studied to overcome it ([1,2] and references therein). Zn metal has a number of characteristics that make it well suited for use as a protective coating for ferrous substrates and as a result Zn coatings are widely used for corrosion control [3– 6]. Their performance results from their ability to react with the atmospheric compounds (O2, CO2 and H2O) which form over the coating surface dense, adherent films, whose rate of corrosion is considerably below that of ferrous materials (barrier protection). Additionally, Zn is anodic to iron and steel and consequently offers cathodic protection. A number of different methods of casting Zn coatings are commercially available. The most widely used is hot-dip galva⁎ Corresponding author. Tel.: +30 2310 998 085; fax: +30 2310 998 003. E-mail address: [email protected] (G. Stergioudis). 0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.11.039 nizing [3–6]. In this case, the ferrous substrate is immersed in a bath of molten Zn and as a result it is covered by a layer of Zn with an average thickness of a few tens of micrometers. The effectiveness of this method is undisputable. However, its high environmental impact imposes the investigation of alternative coating techniques friendlier to the environment. A promising technique that could be used instead of hot-dip galvanizing is chemical vapor deposition (CVD) by pack cementation [7]. In this case, a Zn diffusion coating is formed by heating the substrate up to 400 °C covered with a mixture of powders containing Zn. A similar method is CVD in fluidized bed reactor (FBR) [8]. In this case, the substrate is immersed in a fluidized bed of particles containing Zn and heated up to 400 °C. Although CVD is widely used for the deposition of aluminum, chromium, silicon and other elements on different substrates in industrial-scale facilities [7–17], its application for Zn deposition is relatively rare. As a matter of fact, only the Table 1 Chemical composition (wt.%) of SAE 1010 specimens Fe C Mn S P Si Al Balance 0.11 0.55 0.012 0.016 0 0 G. Vourlias et al. / Surface & Coatings Technology 200 (2006) 6594–6600 Table 2 Pack cementation conditions Zn (wt.%) NH4Cl (wt.%) Al2O3 (wt.%) Temperature (°C) Heating duration (min) 50 2 5 2 5 2 5 2 5 2 5 2 5 2 5 2 5 2 5 48 45 48 45 48 45 48 45 28 25 28 25 28 25 28 25 28 25 250 240 350 240 380 240 400 240 400 240 400 120 400 60 400 30 400 15 50 50 50 70 70 70 70 70 6595 time and powder composition). Furthermore, since hot-dip galvanizing is mainly used for corrosion protection, induced from atmospheric exposure, the anticorrosive performance of the as-casted coatings is studied in a salt spray chamber, where the atmospheric corrosion phenomena are highly accelerated. This investigation is of great importance because it offers important data for the scaling up of these methods to industrial applications. Furthermore, the summary of data concerning the estimated service life of Zn diffusion coatings, as it is determined with accelerated corrosion tests, could assist the preliminary economic assessment of these methods. 2. Experimental procedure formation of a two-step Al and Zn diffusion coating is reported [18]. In this case, a Zn layer formed with sherardizing (mechanical plating [19]) is covered by an Al layer formed by pack cementation. The corrosion resistance of Zn–Al coatings formed with this method has been studied elsewhere [20]. The present work deals with a systematic research on the microstructure of Zn diffusion coatings formed with pack cementation and FBR. The effect of different processing factors is studied (temperature of the coating process, heating Commercial, hot-rolled low carbon steel sheets SAE 1010 were used as substrates for the sample preparation. The chemical composition as analysed by mass spectrometry (SPECTROLAB M8 mass spectrometer) is listed in Table 1. The samples were sized to 15 mm × 10 mm × 2.2 mm and then polished progressively by hand finishing up to 600-grit SiC paper. Some of the as-prepared samples were coated by pack cementation. For that purpose, they were placed in porcelain crucibles filled by powder mixtures with the formulations summarized in Table 2. The crucibles were covered with porcelain lids, sealed with high-temperature resistant cement and placed in a tubular argon-purged electric furnace, which was preheated at the temperature of the coating formation. The heating duration is presented in Table 2. After the heating period, the crucibles were left in the furnace to Fig. 1. SE micrographs of pack coatings formed at 250 °C (a), at 350 °C (b), at 400 °C with Zn concentration 50 wt.% (c), and at 400 °C with Zn concentration 70 wt. % (d). 6596 G. Vourlias et al. / Surface & Coatings Technology 200 (2006) 6594–6600 Element concentration (mol%) 100 δ -FeZn10 γ -Fe11 Zn40 Zn 80 60 (1) 40 20 Fe-Zn Interface 0 0 2 4 Fe 6 8 10 12 14 Fig. 4. SE micrograph of the cross section of a coating formed in FBR at 400 °C with 70 wt.% Zn. 16 Distance from the surface (μm) Fig. 2. Compositional profile along the cross section of a pack coating formed at 400 °C with 50 wt.% Zn after a heating time of 240 min. The peak noted with (1) refers to a Fe–Zn inclusion. cool down to ambient temperature without interrupting the Ar flow. Other samples were coated by FBR using a typical apparatus described previously [8]. The deposition temperature was 400 °C, the powder mixture was composed of 70 wt.% Zn, 5 wt.% NH4Cl and 25 wt.% Al2O3, the heating duration was 1 h and the fluidization gas was Ar. In both processes, the coating is formed from the gas phase in a minimum three reaction steps. The first one includes NH4Cl decomposition to NH3 and HCl. The second refers to the sum of composition reactions where Zn compounds with Cl and NH3 are formed, while the third one refers to the sum of decomposition reactions which lead to the deposition of Zn on the ferrous substrate. Al2O3 does not take part in any of these reactions. This mechanism is the same regardless of the deposition methods used in this work (pack cementation or fluidized bed reactor). 1 1 INTENSITY (ARBITRARY UNITS) The corrosion of the specimens took place in a Salt Spray Chamber SC-450. The corrosive medium was 5 wt.% NaCl in de-ionized water. The chamber temperature was 40 °C and the relative humidity 100%. Specimens were retrieved from the chamber after 24, 48, 96, 144, 192 and 240 h. In the chamber, simultaneously with the pack coated samples, were placed hotdip galvanized samples on the same steel substrate for comparison reasons, which were also retrieved after 24, 48, 96, 144, 192 and 240 h. For the examination of the microstructure and the corrosion behavior, cross sections from each specimen have been cut before and after the corrosion test, mounted in bakelite, polished down to 5-μm alumina emulsion and etched in a 2% Nital solution. The nature of the phases was determined using X-ray diffraction (XRD), while the observation of the coatings took place with scanning electron microscope (SEM) associated with an Energy-Dispersive X-ray Spectroscopy (EDS) analyser. The XRD experiments were done in a conventional powder diffractometer (SEIFERT 3003 TT-CuKα radiation) in Bragg–Brentano geometry; therefore, the XRD patterns originate from the whole coating volume and do not discriminate the contributions from the surface and buried layers. No 2 γ phase δ phase 1 2 1 1 1 1 1 1 1 2 1 1 30 1 1 1 1 2 40 50 2θ degrees 60 1 2 1 2 70 0 Fig. 3. XRD pattern of pack coatings formed at 400 °C after a heating time of 240 min. The peaks noted with (1) refer to γ-Fe11Zn40 phase and the peaks noted with (2) refer to δ-FeZn10 phase. G. Vourlias et al. / Surface & Coatings Technology 200 (2006) 6594–6600 6597 Fig. 5. SE micrographs of the cross section of corroded pack coatings [(a) after 24 h, (b) after 48 h, (c) after 240 h]. contribution from the stainless steel substrate has been recorded in the XRD patterns, as determined by the penetration depth of the X-ray beam following the analysis described in Ref. [21]. For the SEM study, the specimens were examined with a 20-kV JEOL 840A SEM equipped with an OXFORD ISIS 300 EDS analyzer and the necessary software in order to Fig. 6. Chemical mapping of the interface of corroded pack coating after 144 h of exposure [(a) SE micrograph, (b) O distribution, (c) Zn distribution, (d) Cl distribution]. 6598 G. Vourlias et al. / Surface & Coatings Technology 200 (2006) 6594–6600 coating appears to be much worst than expected. Although it is more compact and the uncoated areas on the substrate are much smaller, many cracks remain. At 400 °C coating integrity comparable to the integrity of the hot-dip galvanized coatings (Fig. 1c) has been achieved. The thickness of the coating is homogeneous (about 80 μm in average). This thickness is likely to offer equivalent corrosion protection to the hot-dip galvanised coatings because it is of the same order of magnitude [3–6]. Fig. 2 shows a representative EDS line scan of these coatings, which reveals the distribution of Fe and Zn in the coating cross section. From these data and micrographs similar to those of Fig. 1c and d, it is evident that the coating has a two-layer structure, with a surface layer, ∼3.5 μm thick, enriched in Zn. The Zn concentration, as it is calculated from the diagram of Fig. 2, implies that the buried (inner) layer refers to the γFe11Zn40 while the surface (outer) layer corresponds to the δFeZn10 phases of the Fe–Zn phase diagram, since the Zn amounts to about 20–25 wt.% in the inner and 8–12 wt.% in the outer layer [3]. The formation of these phases was probably accomplished through Zn inward diffusion in the ferrous substrate. If outward diffusion was predominant, inclusions of alumina would be observed in the coating cross section originating from the powders of the pack [7]. The structure of the coating was verified by XRD (Fig. 3). The XRD pattern corresponds exclusively to γ-Fe11Zn40 (peaks denoted by 1) and δFeZn10 (peaks denoted by 2). Several inclusions were also observed in the pack coatings (the composition of such an inclusion is also recorded as the abrupt peak in the EDS line scan of Fig. 2). Their EDS analysis shows that they are composed by Fe and Zn at almost equal concentrations. Consequently, they should refer to a Fe–Zn solid solution, and as a result, no peaks were recorded at the XRD. The above-mentioned phases compose also the pack coatings formed by different formulations and at different heating times (Fig. 1d). The heating time variations seem to affect only Fig. 7. Element distribution at the cross section of a corroded pack coating formed at 400 °C with 50 wt.% Zn after a heating time of 240 min. perform point microanalysis, linear microanalysis or chemical mapping of the surface under examination. 3. Results and discussion 3.1. Characterization of the coatings prior to corrosion Some typical micrographs of pack coatings cross sections prior to corrosion are presented in Fig. 1. From these micrographs, it is obvious that the coating quality formed at 250 °C is very poor (Fig. 1a). The coating thickness is inhomogeneous, large areas of the ferrous substrate are exposed and many cracks are observed. In any case, coatings of that quality are useless for any kind of applications. The coating quality is improved as the temperature increases at 350 °C (Fig. 1b) and 380 °C. However, even in this case, the INTENSITY (ARBITRARY UNITS) 1 γ-phase 3 3 31 11 30 1 3 3 3 3 2 1 40 1 1 50 2θ 3 1 1 3 1 2 3 60 3 70 degrees0 Fig. 8. XRD pattern of the surface of a corroded pack coating formed at 400 °C with 50 wt.% Zn after a heating time of 240 min and 8 days corrosion (1: ZnO, 2: iron oxides, most probably Fe2O3 or FeO, 3: zinc hydrated chlorides). G. Vourlias et al. / Surface & Coatings Technology 200 (2006) 6594–6600 6599 the coating thickness, which decreases as heating duration decreases. The effect of the formulation is not quite obvious. It seems that too low variations of the pack composition are ineffective. The coatings formed in FBR are characterized by the same microstructure than that of the pack coatings (Fig. 4). Two layers are also observed with compositions referring to the δFeZn10 for the surface layer and to the γ-Fe11Zn40 phase for the buried layer. The coating thickness is of the same order of magnitude. However, the FBR-produced coatings are more uniform, since no inclusions of other phases are observed. 3.2. Characterization of the coatings after corrosion The corrosion of the pack specimens was severe. As the micrographs of Fig. 5 show, although after 24 h, no degradation is observed, after 48 h, small cracks appear in the coating. The size and the density of these cracks increases as the exposure time increases, leading finally to the delamination of the coating (Fig. 5c). These cracks are also the preferred paths for the penetration of chloride and oxygen ions in the coating as the chemical mapping of Fig. 6 and the EDS line scan of Fig. 7 shows. The same phenomena were also observed for the coatings formed in FBR. The presence of cracks implies that the initiation of corrosion could be ascribed to a mechanism of stress corrosion [1,2]. In the case of hot-dip galvanized coatings, the mismatch between the thermal expansion coefficients of the Fe–Zn phases of the coating and the iron substrate leads to the development of tensile residual stresses upon cooling, which in the case of galvanizing usually takes place by exposing the coated object in the air. These stresses are relieved through the formation of cracks inside the Zn coating [22]. The crack formation is already accomplished when the coating is exposed in the atmosphere. The coating thus contains a pre-existing crack network, which, as far as it concerns hot-dip galvanized coatings, is accumulated mainly in the δ-FeZn10 phase [22]. Since the outer phase of the Zn diffusion coatings, which are studied in the present work, is the δ-FeZn10, it is likely that the surface of the coating is characterized by a number of crack tips, formed with the same mechanism as it was previously mentioned for the hot-dip galvanized coatings. The cracks offer paths to chloride ions which are dissolved in the corrosive medium. The Cl− ions and oxygen which penetrate into the coating react with Zn [1,2] resulting in the Zn dissolution through the formation of soluble materials as Zn hydrated chlorides (Fig. 8). Furthermore, ZnO is formed (Fig. 8) whose specific volume is much higher than the specific volume of Zn. Consequently, the widening of the cracks is enhanced. These phenomena finally lead to the decomposition of the coating. By contrast, the corrosion of hot-dip galvanized coatings is almost uniform (Fig. 9). This could be explained by the fact that in this case, the δ-FeZn10 phase is not exposed to the corrosive medium. The outer phase of the hot-dip galvanized coatings is the eta (η) phase which is composed by almost pure Zn. The eta phase is characterized by low hardness unlikely to the Fe–Zn phases that form the Zn pack coatings [3]. Consequently, the Fig. 9. SE micrograph of the cross section of corroded hot-dip galvanized coatings after 144 h of exposure. crack propagation in this phase is rare, and as a result, no stress corrosion occurs. However, although the corrosion mechanism differs, the attack of the galvanized coating seems to be also very severe. After 8 days of exposure, a large volume of corrosion products is gathered on their surface, while Zn chlorides and oxides are formed in the coating, as EDS showed. Generally, there is a steady degradation of the coating as the exposure time increases. Hence, it could be deduced that the corrosion performance of the above-mentioned types of Zn coatings is similar. 4. Conclusions The following conclusions have been drawn in this work. 1. The Zn coatings formed by pack cementation and FBR at temperature 400 °C are composed by two layers referring to the γ-Fe11Zn40 and the δ-FeZn10 phases of the Fe-Zn phase diagram. 2. In the pack coatings, inclusions were detected which are composed by Fe and Zn at almost equal concentrations. As far as it concerns these conclusions, further research is needed, in order to reveal their contribution to the mechanical and the anti-corrosive behavior of the coating. 3. The corrosion of the diffusion coatings could be ascribed to a mechanism of stress corrosion. However, in order to draw a conclusion beyond any doubt, a microscopic re-examination of the non-corroded pack and FBR coatings is necessary to verify the formation of cracks during their deposition. 4. The corrosion performance of the Zn coatings studied in this work is almost similar to the corrosion performance of the hot-dip galvanized coatings. Acknowledgments This project was partially financed by the Greek Ministry of National Education through the program Pythagoras II (project no. 80832). References [1] E. Mattson, Basic Corrosion Technology for Scientists and Engineers, 2nd ed., IOM Communications, London, 1996. 6600 G. Vourlias et al. / Surface & Coatings Technology 200 (2006) 6594–6600 [2] M. Fontana, Corrosion Engineering, 3rd ed.McGraw-Hill, New York, 1986. [3] A.R. Marder, Prog. Mater. Sci. 45 (2000) 191. [4] ASM Handbook, Hot Dip Coatings-Corrosion, vol. 13, ASM, New York, 1999. 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